It is known in the art relating to internal combustion engines and in particular to two cycle diesel engines to provide an exhaust driven turbocharger having supplemental means such as a gear drive operative to positively drive the turbocharger at a predetermined minimum speed ratio with respect to engine speed. In this manner, the required minimum air flow is provided for all engine operating conditions wherein exhaust energy provided by the engine exhaust system is insufficient to drive the turbocharger at the required minimum speed.
In many common applications of two cycle diesel engines, such as power for railway locomotives, a mechanical turbocharger drive ratio adequate to provide the required minimum air flow for operation during acceleration and at higher engine speeds and loads is known to provide air flow substantially exceeding that which is required for operating the engine at the lower end of the speed range and at idle. This occurs because substantially lower fuel injection rates are required for operation in the lower ranges of speeds and loads. Thus while adequate engine performance for acceleration and power operation at all engine speeds may be provided by a single speed ratio mechanical drive for a two cycle engine turbocharger, the unnecessarily high air flow provided at idle and low operating speeds and loads requires a greater than necessary amount of energy to drive the turbocharger with resulting adverse effects on fuel economy in the lower speed ranges.